INPUT LEVEL

Adjusts the signal going into the processor. To avoid clipping the input, don’t slam a compressor with too much input, even though this gives more compression; for more compression, reduce the Threshold control and/or increase the Ratio control.

THRESHOLD

Sets the level above which signals will be compressed. With a lower threshold, more of the signal crosses this threshold, compressing more of the signal. Once the signal drops below the threshold, the compressor leaves the signal alone until it exceeds the threshold again.

RATIO

Sets how the output signal changes in relation to the input signal, once the input signal exceeds the threshold. For example, with a 4:1 compression ratio, if the input increases by 8dB,  the output increases by only 2dB.

ATTACK

Determines how long it takes for the compression to kick in once the signal goes above the threshold. Longer attack times let more of a signal’s natural dynamics through, but those signals are not being compressed. If it’s really important to clamp peaks, a short attack time works best.

DECAY (ALSO CALLED RELEASE)

Sets the time required for the compressor to “let go” of the signal once the input passes below the threshold. With short release times, the compressor tracks even very slight level changes, but this can produce a “choppy” sound; turn up the release time for a smoother sound.

AUTO

If present, an “auto” control automatically sets appropriate attack and decay times, based on the incoming level.

OUTPUT CONTROL

Compressing peaks reduces the overall peak level. Increasing the output compensates for the volume drop. Turn this control up until the peak levels of the compressed signal match the peak levels of the bypassed signal; in other words, there’s unity gain between the input and output.

AUTO MAKE-UP GAIN

This function automatically increases the output gain to make up for any signal level loss from compression.

MIX

While not a common parameter, this sets a blend of processed signal and input signal. The input signal contributes the original transients to give a “punchier” sound.

HARD KNEE/SOFT KNEE SWITCH

With soft knee, when the input exceeds the threshold, the compression ratio is less at first, then increases up to the specified ratio as the input increases. With hard knee, as soon as the input signal crosses the threshold, it’s subject to the specified compression amount. The hard knee action creates more punch; soft knee may work well if the level variations are extreme, as this better preserves the sense of dynamics. While typically a switch, this also can be a continuously-variable parameter.

SIDE CHAIN JACKS

These let you insert filters in the compressor’s feedback loop to restrict compression to specific frequency ranges. Another use is to trigger compression from a different audio source.

LINK SWITCH

With stereo compressors, this changes the operating mode from dual mono to stereo. Linking the two channels together allows changes in one channel to affect the other channel, which is necessary to preserve the stereo image.

GAIN REDUCTION METER

Indicates how much the signal is being compressed at any given moment. Watch the gain reduction amount carefully to avoid overcompression.

OUTPUT LEVEL METER

Shows the signal level coming out of the compressor, and might be switch-selectable to monitor the input as well. The output level can show how much overall output level you’ve lost by adding compression, which makes it easier to set the output level control correctly.

PEAK/RMS METER SWITCH

Determines whether the meters will display peak or RMS levels.

PEAK/RMS DYNAMICS RESPONSE SWITCH

Sets whether the dynamics control responds to peak or RMS levels.

LIMITER SWITCH

Clamps all peaks to a particular level, usually 0dB. This can be useful with longer attack times, as the attack time may let peaks through that without limiting, could overload your system.

TYPE

A compressor may offer different sonic characters, such as “vintage,” “optical,” and the like. These typically alter the compression curve, or some other aspect of the sound.

BYPASS

Takes the effect in or out of the signal chain. Use this switch often to compare the processed and straight sounds.
Some dynamics processors include dynamic expansion options as well as compression and limiting. Expansion helps minimize noise by reducing low-level signals like hiss. If present, you’ll find controls like the following.

EXPANDER THRESHOLD

Adjusts the level below which signals will be expanded (i.e., the output level drops off at a faster rate than the input level). Once the signal goes above the threshold, then the processor leaves the signal alone until it goes below the threshold again.

EXPANDER RATIO

Sets how the output signal changes in relation to the input signal, once the input signal goes below the threshold. For example, with a 4:1 expansion ratio, if the input decreases by 2dB,  the output decreases by 8dB.
Some dynamics processors include noise gates, which minimize noise by reducing or muting the signal level when the signal goes below a certain threshold. If present, you’ll find controls like the following.

GATE THRESHOLD

Sets the level below which signals will be attenuated or muted. Once the signal goes above the threshold, then the noise gate leaves the signal alone until it goes below the threshold again.

GATE ATTENUATION

Determines how much the signal is reduced once it goes below the threshold. This may mute the signal completely.

GATE ATTACK

Adjusts how long it takes for attenuation to occur after the signal goes below the threshold.

GATE DECAY

Determines how long it takes for the signal to go from the attenuated condition to normal after the signal rises above the threshold.

GATE ON/OFF INDICATOR

Shows whether the gate is in an “open” or “closed” (attenuation active) condition.